The Process of Aluminum Extrusion

Abstract:

The process of aluminum extrusion is very important for a variety of industries, and there are many factors that impact how successful the extrusion can be. For the purpose of this lab, the two variables analyzed were the die diameter and the stroke rate. As can be seen in the figures mentioned later in this report, the microstructure at different stroke rates affects the material properties such as flow stress in the aluminum. Altering the stroke rate can change what a certain extrusion will be used for, and how well it will serve its purpose. The flow stress was severely greater for the smaller stroke speeds because they allowed the aluminum sample to cool, which made it harder to work the material. The grains in the center of the aluminum elongated much more as the stroke rate was increased. Decreasing the size of the die also increased the flow at the center of the aluminum rod following extrusion.

Introduction:

Extrusion is a compression process where a metal is forced to flow through a die opening that results in a desired cross sectional shape. It was first used as an industrial process during the industrial revolution in the early 1800’s in England. Some standard parts manufactured using extrusion are trim parts used in automotive and structural applications, window frame members, and aircraft structural parts (Udomphol, 2007). Extrusion is carried out with various methods that vary depending on the material and the use of the product. There are two processing methods within extrusion: direct extrusion and indirect extrusion. Direct extrusion (forward extrusion) involves a metal billet inserted into a container, and the ram compresses the material, forcing the extrusion through the die. For indirect extrusion, the die is mounted to the ram as opposed to the opposite end of the container. The temperature of the extrusion is another factor to take into consideration. It depends on the work metal and the amount of strain it is to be subjected to during compression. Most metals undergo hot extrusion because of the large amount of forces required (Udomphol, 2007). Aluminum is widely considered the most ideal metal for extrusion both hot and cold, and there are many consumer products made from both processes (Groover, 2013). For the purposes of this lab, a direct, hot extrusion was executed with the billet heated to 500oC. (Metal Extrusion Laboratory, 2016).

The two primary variables that were address in the aluminum extrusion lab were the stroke rate (cm/s) and the die hole size. There were five different dies that were used with holes ranging from x-x. As to be expected, as the die size decreased, the extradite length increased as a result of the billet pushed through a smaller hole. The largest issue that can arise is the die can get stuck because of the excess metal that builds up and then cools. A similar issue prevented the group from collecting data on a few of the dies, so for the purpose of this lab the data from the first section. For the strain rate, the goal was to set the strain rate to approximately 0.2, 0.3, 0.6, and 0.7 cm/s. These tests were much more successful and will be discussed.

There are many important parameters that must be discussed and considered. Most of the variables referenced in the parameters can be seen in Figure 1 below. The figure assumes the final product and billet have a circle cross-section.

The first parameter, the extrusion ratio or reduction ratio is defined as:

rx=AoAf

Where rx is the extrusion ratio, Ao is the cross-sectional area of the starting billet, and Af is the final cross-sectional area of the extrusion. Rx can be used to determine the true strain in the extrusion.

A variety of methods have been developed to calculate the true strain and the associated ram pressure in extrusion. The primary one used for the purpose of this lab is Johnson’s equation, which is:

x=a+b lnrx

Where ex is the extrusion strain, and a and b are empirical constants for a given die angle. Some typical values for these constants are a=0.8 and b=1.2-1.5. These constant values typically increase with increasing die angle.

Fig. 1: Diagram of Direct Extrusion

Experimental Procedure:

The safety equipment necessary for this lab includes safety classes and protective gloves. When the pump is on for the extruder, it is important to keep hands and face away from the machine. The protective gloves are essential due to the nature of hot extrusion, so the billet, die, and extradite will be hot following the full process.

Before beginning the tests, the dimensions of the five dies and the dimensions of a cold billet are to be measured. Using the socket wrench, the jam nut and three core bolts are loosened and the proper die is inserted with the conic side facing in. Once the die is fully inserted, the jam nut and three core bolts are tightened by turning each a half turn at a time. Then lift the extradite receiver yoke and place the pin lock in the hole. Once everything is set, turn on the machine using the black lever and take note of how to turn the pump on and off. Using the ruler that is parallel to the ram, measure the speed of the machine using a stopwatch. Using the lever in the back, adjust the stroke rate appropriately. After recording